As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and operation potential, long cycle lifespan, and low self discharge rate, which is now commercialized and widely used.
An electrode assembly having a cathode/separator/anode structure, which constitutes a secondary battery, may be generally classified as a jelly-roll (wound) type electrode assembly or a stacked type electrode assembly based on the structure of the electrode assembly. The jelly-roll type electrode assembly is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and helically winding the anode/separator/cathode structure. The jelly-roll type electrode assembly is suitable for a cylindrical battery; however, the jelly-roll type electrode assembly is not suitable for a prismatic battery or a pouch-shaped battery because the electrode active material is separated or space utilization is low. On the other hand, the stacked type electrode assembly is configured to have a structure in which a plurality of unit cathodes and a plurality of unit anodes are sequentially stacked. The stacked type electrode assembly has an advantage in that the stacked type electrode assembly can be configured to have a prismatic structure; however, the stacked type electrode assembly has disadvantages in that a process for manufacturing the stacked type electrode assembly is complicated, and, when external impact is applied to the stacked type electrode assembly, electrodes of the stacked type electrode assembly are pushed with the result that a short circuit may occur in the stacked type electrode assembly.
In order to solve the above-described problems, there has been developed an improved electrode assembly which is a combination of the jelly-roll type electrode assembly and the stacked type electrode assembly, i.e. an electrode assembly configured to have a structure in which full cells having a cathode/separator/anode structure of a predetermined unit size or bicells having a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure of a predetermined unit size are folded using a long continuous separator film. Examples of such an electrode assembly are disclosed in Korean Patent Application Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application. In the present application, the electrode assembly with the above-stated construction is referred to as a stacked/folded type electrode assembly.
A secondary battery formed to have a structure in which the stacked type electrode assembly or the stacked/folded type electrode assembly is mounted in a battery case may be configured in various forms. A representative example of the secondary battery is a lithium ion polymer battery (LiPB) using a pouch-shaped case formed of an aluminum laminate sheet.
The lithium ion polymer battery (LiPB) is configured to have a structure in which an electrode assembly manufactured by thermally welding electrodes (cathodes and anodes) and separators is impregnated with an electrolyte. Mostly, the lithium ion polymer battery is configured to have a structure in which the stacked type electrode assembly or the stacked/folded type electrode assembly is mounted in a pouch-shaped battery case formed of an aluminum laminate sheet in a sealed state. For this reason, the lithium ion polymer battery is often referred to as a pouch-shaped battery.
Generally, a device for folding an electrode assembly through a rotational motion is used to fold the stacked/folded type electrode assembly. Referring to FIG. 1, the folding device includes a web supply unit 400 formed in the shape of a roller to supply a web 200 having plate-shaped unit cells 100, 101, 102 . . . arranged at the top of a separation film at predetermined intervals and a winding jig 300 to rotate the unit cells while holding a first one of the unit cells of the web so that the unit cells are sequentially stacked in a state in which the separation film is disposed between the respective unit cells. As the winding jig 300 is rotated, the unit cells 100, 101, 102 . . . are sequentially stacked.
Since the winding jig 300 is rotated to wind the plate-shaped unit cells 100, 101, 102 . . . , however, tension of the web 200 may be changed. The change in tension of the web 200 badly affects overall process before the web 200 is supplied to the winding device. In order for the winding device to uniformly maintain tension of the web 200, therefore, a method of compensating for the position of the winding jig 300 in an advancing direction of the web 200, i.e. in an X-axis direction, may be considered.
In connection with this matter, referring to FIG. 2, on the assumption that the turning radius of the winding jig is a, the distance from the roller to the center of rotation of the winding jig is b, and the length of the web from the roller based on an angle of the winding jig is c, c may be represented by the following equation based on the change of an angle θ of the winding jig with respect to an X axis.c=(a2+b2−2ab cos θ)1/2 
FIG. 3 is a graph showing a change amount of the c value based on the angle θ and a length change amount (linear change amount) of the web in a linear case. In FIG. 3, since the angle θ is rotated at uniform velocity, angular velocity is uniform, and therefore, the graph may be identical to a time-displacement graph. Consequently, the linear change amount indicates uniform velocity as a straight line having a uniform inclination.
The rotational motion of the plate-shaped structure is not changed into a linear change amount. Consequently, it is necessary to perform compensation based on the difference between the change amount of the c value based on the angle θ and the linear change amount. A graph of the compensation amount is also shown in FIG. 3.
In a case in which the compensation amount obtained using the above calculation method is applied to compensate for the position of the rotary shaft of the winding jig in the advancing direction of the web (X-axis direction), however, it is difficult to perform the process at predetermined rotational velocity or more.
In connection with this matter, referring to FIG. 4, the graph of the compensation amount has a problem in that differentiation is not possible at a point at which θ is 180 degrees, and jerk is excessive at the point. This problem is not serious in a general production process, which is in use. In a case in which the rotational velocity of the winding jig is increased, however, an excessive jerk is applied to the winding device with the result that it may be necessary to frequently replace components and products may be defective. Particularly, in a case in which the rotational velocity of the winding jig is increased twice or more than that in the general production process, which is in use, to improve process efficiency, more serious problems are caused.
Therefore, there is a high necessity for a technology that is capable of solving the above problems.